The present invention relates to a method in which a firing angle and/or an ignition time of a gasoline or diesel engine having direct injection is regulated with the help of a knock control for preventing knocking during engine operation. In fault-free operation of a knock control, fuel is injected once per operating cycle into the combustion chamber of a cylinder of the engine. In addition, combustion is monitored by the knock control.
A known measure in the event of failure of knock control of gasoline engines is a safety retardation of the firing angle relative to the crankshaft position, the cylinder standing at 0xc2x0 (crank angle) at top dead center, TDC. The firing angle is typically retarded by 12xc2x0 to 15xc2x0 (crank angle). At such a firing angle, engine knocking does not occur. However, one disadvantage of this is the loss of engine sufficient due to the safety retardation.
The problem on which the present invention is based is to provide a simple method of preventing knocking in the event of failure of a knock control, so that it will also permit operation of the engine at a high efficiency even in the event of failure of the knock control. In addition, a device is also to be provided for this method.
It is already known from Japanese Patent Application No. 56 066,426 to increase the quantity of fuel injected as a knock-reducing measure when a knock sensor fails.
The relationships in direct fuel injection are utilized for the present invention. Due to the design of the injection system, fuel is atomized in an especially fine spray. Different rotational movements of air layers are achieved due to the air flow created in the intake and compression strokes. There are different mixing ratios in the combustion chamber due to the injection of fuel in the intake stroke as well as the compression stroke. Around the spark plug there is a rich fuel-air mixture surrounded by lean layers at the edge of the combustion chamber. Lean layers, i.e., layers in which the mixing ratio of fuel to air in kg is less than 1:14.8, are not as flammable as layers in which the fuel mixture is rich. A fundamental cause of knocking is spontaneous ignition in boundary areas occurring in addition to the main ignition.
The present invention is based on the finding that whereas the knocking tendency of direct injection engines is already reduced, it may be further decreased by multiple injection and/or a different injection quantity during a operating cycle of the engine. For example, in the case of a gasoline engine, there is a single injection during the intake stroke and the compression stroke or the injection quantity is increased. This measure yields a very large gradient of the mixing ratio from the spark plug area to the edge of the combustion chamber. This gradient counteracts engine knocking. Since knocking is already limited in this way, it is no longer necessary to make a safety retardation or at any rate a smaller angle may be used for the safety retardation. A considerable loss of engine efficiency may be prevented by the method according to the present invention, because the firing angle need not be retarded by the usual large crank angle values of 12xc2x0 to 15xc2x0.
In a refinement of the method according to the present invention, the firing angle is controlled as a function of the engine rotational speed and/or engine load after the change in injection quantity. The engine rotational speed and engine load are the essential characteristic variables which influence the firing angle. However other characteristic variables may also be taken into account to improve the control.
The present invention also relates to a device for preventing knocking in the event of failure of a knock control. This device makes it possible to implement the method steps of the method according to the present invention and it includes a monitoring circuit for a knock control and a control unit which initiate substitute measures to prevent knocking in the event of a fault in the knock control, depending on the output signal of the monitoring circuit. The control unit is designed so that the substitute measure in the case of a gasoline engine involves, for example, a transition from a single injection of fuel per operating cycle of a cylinder of the engine to two injections of fuel per operating cycle. The technical effects and considerations described above for the method according to the present invention also apply to the device according to the present invention.
In a next refinement, the control unit controls the firing angle as a function of the engine rotational speed and/or engine load. Thus the essential characteristic variables which are crucial for the adjustment of the firing angle are also taken into account with appropriate measure. Although regulation in a closed-loop control circuit is no longer possible, control is performed nevertheless to be able to adjust an acceptable firing angle.